A number of different analog-to-digital (A/D) converter types are known in the prior art. These include the dual ramp or slope A/D, also known as an integrating A/D; the successive approximation A/D; the tracking (counter-comparator) A/D; and the multi-comparator or flash A/D. For completeness, the voltage to frequency converter, when used with a frequency counter, may also be considered as a type of A/D converter.
Each of the foregoing A/D converter types exhibits certain advantages and also certain disadvantages. For example, the dual slope A/D is a relatively low power and inexpensive device, although it is also relatively slow compared to the other types. The successive approximation A/D can exhibit high speed conversion, although high resolution can be achieved only with precision voltage comparators and, hence, considerable expense. The tracking A/D is also a high speed device, but is known to be susceptible to noise. The flash A/D generally exhibits the fastest conversion speed, but is also expensive to implement when high resolution is required.
In general, when high accuracy and resolution is required all of these A/D converter types require the use of precision voltage comparators and associated circuitry. This requirement increases both the complexity and expense of the conversion circuit.
Monotonicity is one important operating characteristic of an A/D converter, and refers to an ability of the A/D converter to convert a linearly increasing analog input signal without exhibiting missing or erroneous digital values or codes at the output.
Other operating characteristics of an A/D converter that become important when it is desired to integrate one or more A/D converters into an integrated circuit assembly include the operating power and the manufacturability of the circuit.
For example, when it is desired to integrate an A/D converter into a readout integrated circuit for use with a focal plane array (FPA) of radiation detectors, such as a one or two dimensional FPA of infrared (IR) detectors, the power consumption (and heat dissipation) become important because the FPA must typically be cooled to, and maintained at, cryogenic temperatures during operation. As such, it is desirable that the A/D converter not add significantly to the heat load of the overall imaging system. Any additional heat load is reflected in more power being required in the cryogenic cooler, in addition to an increase in the weight and size of the cryogenic cooler.
Manufacturability is also an important concern for this application in order to reduce fabrication and testing costs. For example, the parallel type of A/D converter typically employs a tapped resistor network and an array of comparators. However, in order to achieve, by example, ten bit accuracy, as many as 1024 matched precision resistors and 1023 high-speed comparators, in addition to a fast logic array to decode the outputs of the comparators, may be required. As can be realized, the integration of such a complex circuit, with its required precision components, would not be cost effective for many applications.
Furthermore, inherent limitations in the foregoing A/D types include a fixed resolution that is set by the number of output bits, and an output error that results from rounding off the digital output to the nearest least significant bit (LSB). This latter limitation prevents the extraction of any additional bit resolution from the output data stream.